1. Field of the Invention
The present invention relates to a film bulk acoustic resonator and a method of manufacturing the same, and more particularly, to a film bulk acoustic resonator and a method of forming the same having a thermal oxidation layer and a stable resonating structure easily formed by a dry etching process.
2. Description of the Related Art
Recently, components, such as a radio frequency related component, used in a mobile communication terminal have been rapidly improved according to a tendency toward minimization and multi-function of the mobile communication terminal. A filter, one of critical components in the mobile communication terminal, performs a function of filtering a predetermined signal or selecting a desired signal in radio frequency signals.
Particularly, as a frequency band used in the mobile communication terminal becomes higher, a component used for an ultra (super) high frequency band is required. However, the super high frequency band component is difficult in minimizing in size and lowering a manufacturing cost. For example, a conventional dielectric resonator and filter used in the frequency band of more than 1 GHz cannot be integrated into the minimized and multi-functional mobile communication terminal due to the bulky size and shape of the conventional resonator and filter.
Although an acoustic surface resonator or crystal resonator replacing a dielectric resonator has been introduced, an insertion loss is still greater than the conventional resonator, and it is impossible to integrate and minimize the acoustic surface resonator in a desired size and volume. Moreover, the manufacturing cost is not decreased.
In an effort to solve the above problems, a film bulk acoustic wave resonator (FBAR) and a thin film resonator (TFR) using thickness vibration of a piezoelectric layer have been recently proposed.
In the FBAR, a thin layer made of a dielectric material, such as ZnO or A1N, is formed on a semiconductor substrate, such as silicon and GaAs, to generate a resonance using a piezoelectric characteristic of the thin layer. It is possible to reduce a manufacturing cost and a weight of the thin layer and to maintain a high quality of the thin layer. The thin layer can be used in wireless communication apparatus and equipment having the super high frequency band of 900 MHz-10 GHz. The thin layer is minimized ten times less than the dielectric filter and has the insertion loss less than the surface acoustic wave component.
Generally, the FBAR is formed with a lower electrode, a piezoelectric layer, and an upper electrode formed on a silicon substrate in order. The silicon substrate is prevented from being affected by the a bulk acoustic wave generated in the piezoelectric layer due to magnetic fields formed by the upper and lower electrodes.
The FBAR requires an additional separating structure separating the silicon substrate from a resonating (activation) area of the upper electrode, the piezoelectric layer, and the lower electrode to improve the electric efficiency, such as the insertion loss and a transmitting gain, and a manufacturing process of the FBAR. The separating structure of the FBAR is classified into a reflective layer structure using the Bragg reflection and an air gap structure having an air gap between the silicon substrate and the resonating (activation) area of the upper electrode, the piezoelectric layer, and the lower electrode.
FIG. 1A is a cross-sectional view of a conventional FBAR having a reflective layer, and FIG. 1B is a cross-sectional view of another conventional FBAR having an air gap.
Referring to FIG. 1A, the FBAR includes a substrate 11, a reflective structure having first and second reflective layers 12a, 12b, and a resonator (activation) area formed by a first electrode 17, a piezoelectric layer 18, and a second electrode 19. The substrate 11 is separated from the resonator area of the first and second reflective layers 12a, 12b having a difference in an acoustic impedance. The first and second reflective layers 12a, 12b are repeatedly formed in the reflective structure to separate the substrate 11 from the resonator area. The reflective structure is the separating structure using the acoustic difference between the first and second structure disposed in a lower portion of the resonator area. This is called a solidly mounted resonator (SMR).
However, the first and second layers 12a, 12b should have the same thickness as a quarter wavelength of a resonating frequency, and a stress generated between the first and second layers should be considered when the first and second layers 12a, 12b are repeatedly formed in the reflective structure. Accordingly, a manufacturing process becomes complicated, and the manufacturing cost is increased. In addition, the SMR is lower than the air gap method in reflective characteristics and limited in being implemented as a resonator due to a decreased effective bandwidth.
In an effort to overcome the above problems, an FBAR having the air gap according to an air gap bridge method has been introduced. According to FIG. 1, the FBAR forms a sacrificing layer on a substrate 21 to form an air gap A1. A first electrode 27 and a piezoelectric layer 28, and a second electrode 29 are formed in order after a membrane layer 25 having an insulation layer is formed on the substrate 21. The sacrificing layer is removed by being etched through a via hole, and the air gap A1 is finally formed. Although the FBAR having the air gap A1 is easily manufactured and has the reflective characteristics better than the SMR, the structure of the membrane layer is deformed and separated from the substrate 21 during forming the FBAR having the air gap because the membrane layer is very unstable.
In order to solve the above problems, an additional layer is provided to support the membrane layer, and another layer is formed to surround the membrane layer of the FBAR. A cross-sectional view of the FBAR is shown in FIG. 2A. FIG. 2B is a plan view of the FBAR of FIG. 2A.
According to FIGS. 2A and 2B, the FBAR includes a substrate 31, a supporting layer 35 formed on the substrate 31 to include an air gap A2, a membrane layer 36 formed on the supporting layer 35, and a resonator (activation) area formed by a first electrode 37, a piezoelectric layer 38, and a second electrode layer 39 formed on the membrane layer 36 in order. The supporting layer 35 supports the membrane layer 36 and includes the air gap A2. The FBAR as shown in FIG. 2A, prevents the membrane layer 36 from being deformed and separated from the substrate 31 during forming a via hole H and removing a photo resist by strengthening a structure of the membrane layer 36. This enables the FBAR to be relatively stable and solid.
However, since the conventional FBAR of FIGS. 2A and 2B includes the substrate made of silicon doped with impurity to have a high electric conductivity, a high frequency of more than 1 GHz may be transmitted to the substrate 31 from the activation area of the first electrode 37, the piezoelectric layer 38, and the second electrode layer 39. As a result, characteristics of the conventional FBAR deteriorate when the FBAR is implemented in an integrated circuit operating in the high frequency band.
After an etching operation is performed to form the air gap A2, the FBAR may become unstable due to a bonding force exerted on the membrane layer 36 having the support layer 35 surrounding the air gap A2. An etchant solution effects the membrane layer 36 because of the bonding force of the etchant solution which is used to remove the sacrificing layer formed with metal oxide, such as ZnO, or a metal, such as Al, Cu, and NiFe.
In addition, when the etchant solution is supplied to the sacrificing layer, the etchant solution may etch the piezoelectric layer 38 when a via hole H is formed to couple the sacrificing layer and an outside of the FBAR. Therefore, the via hole H should be formed on an outside area of the activation (resonator) area. When the via hole H is formed on a corner portion of the activation area, at least four via holes H should be formed on each corner portion of the activation area. In the conventional FBAR, the number of via holes H and a location of the via holes H are very limited. As a result, the number of the vie holes H is increased, the via holes H formed on a specific position of the FBAR badly effects the characteristics of the FBAR.
Although an undercut of the FBAR may be prevented in a conventional sacrificing layer forming process using an dry etching operation of the photo resist after a wet etching operation of the sacrificing layer, this process is very complicated, and the undercut is not easily controlled. If a wing tip is generated due to a lower side profile angle of the photo resist during the above process, the structure of the FBAR becomes weak and easily deformed. According to FIGS. 3A and 3B, a sacrificing layer 43 is formed on a substrate 41, a photo resist layer 44 having the low side profile angle θ1 is formed on the sacrificing layer 43 to form an air gap area, and a sacrificing area 43′ is formed corresponding to the air gap by etching the sacrificing layer by reactive ion etching (RIE). The lower side profile angle θ1 of the photo resist layer 44 is lowered than the conventional photo resist because a material of the photo resist layer 44 has a higher etching ratio than that of the sacrificing layer 43. Accordingly, the wing tip is generated due to the lowered side angle of the sacrificing layer when the photo resist layer 44 is lifted off from a membrane layer after a membrane layer forming operation is performed. In addition to the above problems, the resonator frequency is not easily controlled in the above conventional FBAR.